It will be a failed gearbox of a shootbolt locking mechanism, allowing one side to retract and not the other. A samll window i would suggest deglazing and trying to wedge the side that moves as far open as possible ( so it clears the rest of the frame ) , and then the side that is stuck lever it towrds the other side. Be very wary of how much pressure you are using as you don't want to bend the window